Design Patterns of Ventilatively Cooled Buildings in Denmark - Simple Predesign Tool for Estimation of VC Potential

Abstract

Det forventes, at den igangværende tendens, omhandlende forbedring af bygningers isolering, global opvarmning og stigning i ”heat island” effekten i bymæssige områder, vil forhøje behovet for køling af bygninger i fremtiden. Brugen af direkte ventilative cooling (VC) er en effektiv løsning til at minimere overophedning, samt reducere behovet for mekanisk ventilation. Dette afgangsprojekt har til formål at finde generelle design mønstre for bygninger kølet med VC i Danmark, samt undersøge om det er muligt at estimere et realistisk potentiale for VC i løbet af den tidlige designfase, vha. simple design værktøjer. For at kunne karakterisere generelle design mønstre for bygninger kølet med VC i Danmark, er der lavet en database bestående af bygninger med VC. Ud fra denne database er forskellige parametre med høj betydning for VC observeret. Både nye og renoveret bygninger er analyseret for at finde frem til de forskellige arkitektoniske design og ventilations elementer, der har en betydning for de enkelte løsninger. For at kunne fremme brugen af VC i nye og renoveret bygninger, er to forskellige estimeringsværktøjer analyseret. Disse VC estimerings værktøjer er tiltænkt at blive brugt i den tidlige beslutningsfase. Det første værktøj er EURAC VC potential tool, introduceret af NIST \citep{emmerich}, som er baseret på steady-state beregninger. Det andet værktøj, som er udviklet af denne projektgruppe, er baseret på en 5R1C-model ved brug af dynamiske varmebalance beregninger. De to værktøjer er afprøvet på en testbygning, Aarhus Kommunes nye kontorbyggeri. Resultaterne er verificeret vha. BSim. Efterfølgende er værktøjet, som er baseret på 5R1C-model, afprøvet med forskellige inputs for at undersøge dens robusthed. Det originale EURAC VC potential tool underestimere potentialet for VC, mens den modificeret udgave overestimere. Det kan derfor konkluderes, at steady-state metoden, med konstant belastning samt manglende påvirkning fra termisk masse, ikke kan give en troværdig estimering af potentialet for VC. Derimod er det observeret, at 5R1C-modellen giver en troværdig estimering af potentialet for VC. Resultaterne er under forskellige forhold altid i god overensstemmelse med resultaterne fra BSim. Dog ses det, at 5R1C-modellen underestimere antallet af timer med overophedning ift. BSim. Dette skyldes højst sandsynligt den simple måde at håndtere bygningens termiske masse på i beregningen. Undersøgelse af metodens robustheden viste, at forskellen i resultaterne minimeres, når belastningerne stiger. Det kan dog siges, at VC værktøjet baseret på 5R1C-modellen, kan give troværdige estimeringer af potentialet for VC i kontorbygninger af forskellige størrelser og kompleksitet.

It is expected that on-going tendency of improving building insulation together with global warming and increasing in heat island effect in urban environments will lead to considerable increase in building cooling demand in the future. An efficient solution to minimise the room overheating and reduce the need for mechanical cooling is utilisation of direct ventilative cooling. This master thesis is conducted to observe the general design patterns of ventilatively cooled buildings in Denmark and to investigate the possibility of generating a reliable estimation of ventilative cooling (VC) potential during the predesign phase by the use of simple design tools. To determine the characteristic design patterns of ventilatively cooled buildings in Denmark, a database of buildings using VC is created. Parameters, which are of high importance when designing VC systems, are observed for each building case. Both, new and renovated buildings with VC are analysed to see which special architectural and ventilation system design elements are used. In order to facilitate the implementation of VC in newly designed as well as renovated buildings, two simple VC potential tools are analysed. These VC potential tools are intended to be used in decision making process during the building predesign phase. The first one is EURAC VC potential tool which is based on a steady-state VC potential calculation method originally introduced by NIST. Another tool, which is developed by this project group, is based on 5R1C-model using dynamic heat balance calculation. The two VC potential tools are tested on selected case study building, Aarhus municipality office building. Calculation results are verified by BSim model. Afterwards, the VC potential tool based on 5R1C-model is subjected to a variety of different input conditions to analyse its robustness. It was observed that the original EURAC VC potential tool tends to underestimate the VC potential whereas the modified version of this tool noticeably overestimates the VC potential. Therefore, it can be argued that steady-state methodology with constant heat loads and ignorance of effect from building thermal mass cannot generate reliable estimation of VC potential. In contrast, the 5R1C-model has proven to yield a reliable estimation of VC potential. The results in all variations (control strategy, thermal mass, ACR, NTV) of the case study office are always in a good correlation with the results from the BSim simulations. It also revealed that 5R1C-model slightly underestimates the number of overheating hours. The reason for that could be the simple approach of dealing with the thermal mass in 5R1C-model.The investigation of robustness shows that the deviation in overheating hours between the 5R1C-model and BSim has a tendency to decrease with the increase in heat loads. Thus, one may argue that VC potential tool based on 5R1C-model is able to generate reliable prediction of VC potential in office buildings of different size and complexity.

A master thesis from Aalborg University

Design Patterns of Ventilatively Cooled Buildings in Denmark - Simple Predesign Tool for Estimation of VC Potential